This invention relates to the field of devices for transmitting acoustic energy through a water medium. More particularly it concerns an apparatus for projecting acoustic energy through a water medium that is compact and fabricated in such a manner as to present a cost effective, expendable sound source. In still greater particularity the improved apparatus for projecting acoustic energy employs a d.c. motor that imparts simultaneous outward and simultaneous inward displacements of a pair of rigid pistons to produce a desired arbitrary acoustic waveform output when a coaxially contained shaft rotates suitably coupled cams.
Expendable sound sources have been around in one form or another for quite some time. Explosive charges are still an effective, albeit a primitive, way of radiating acoustic energy through the water. One procedure calls for detonating the explosive at a preselected depth to generate a shock wave which is monitored some distance from the source. An obvious disadvantage of such an approach is that the single shock wave does not lend itself to an extended use or analysis. The uncertainty in depth of detonation makes a series of dropped charges unreliable and the amplitude and frequency content of the explosion generated signals make absolute measurements difficult. Other obvious drawbacks are the numerous hazards associated with ordnance which tend to limit their applications and pose handling, stowage and deployment problems. Researchers agree that collecting acoustic data from the ocean is difficult using explosives and the advantages of the high energy, low cost generation of sound waves that explosives offer are often outweighed by other considerations.
The more sophisticated measurements required for meaningful data by researchers as well as for national defense applications by the military call for sound sources that are able to provide acoustic pressure waves which have prolonged duration and more precise frequency structure. The family of transducers employing ferroelectric driving elements or magnetostrictive could be ideally suitable; however, their relatively high per unit cost for low frequency applications makes them a poor choice for an expendable use. Furthermore, the power requirements tend to be excessive at low frequencies due to inefficient conversion of electric to mechanical energy and thereby prevent a self-contained power supply.
A low frequency sound source that could find use as an expendable source was disclosed by George O. Pickens in his U.S. Pat. No. 4,353,120. His transducer was intended to be used as an apparatus for calibrating or otherwise determining the low frequency operational parameters of an elongate hose-like array. A motor drives a bell crank arrangement or linear motors to reciprocate flexible caps that were spaced away from a test array. The reciprocal displacement of the flexible caps causes the radiation of acoustic energy and the bell and crank arrangement, or linear motors, displaced the caps through the vehicle of a fluid filling elongate, outwardly reaching tubes.
The sound source of Pickens, although not specifically stated as being an expendable source, contains relatively unsophisticated components that may make it cost effective. Since acoustic coupling to the water was indicated as being satisfactory for calibration purposes, it could serve as a sonic source; however, a number of modifications may be desirable.
Thus, there is a continuing need in the state-of-the-art for a cost effective expendable sound source that uses reliable, tested components to project an arbitrary waveform from a compact package.